1. Field of the Invention
The present invention relates to a charged particle beam exposing apparatus configured to form a fine pattern on a sample, and more particularly to a charged particle beam exposing apparatus having a dynamic focus correction function of an optical system devoted to converging, deflection or the like of a charged particle beam.
2. Description of the Related Art
In an electron beam exposing apparatus configured to form a fine pattern on a semiconductor wafer or an optical mask, there is employed a variable shaped beam method or a character projection method which uses a circular beam or generates a beam having a cross section of a rectangle, a triangle or an arbitrary pattern for lithography.
FIG. 1 shows an example of a prior art electron beam exposing apparatus adopting the variable shaped beam method. In the drawing, reference numeral 10 denotes an electron gun; 11, a first shaping aperture; 12, a first projection lens; 13, a shaping deflector; 14, a second projection lens; 15, a second shaping aperture; 16, a reduction lens; 18, an objective deflector; 19, an object lens; 20, a sample base (sample plane); and 21, a reduced image.
In this apparatus, a beam having a desired shape is formed by varying the optical overlap of the first shaping aperture 11 and the second shaping aperture 15 by using the shaping deflector 13, and a position of the reduced image 21 is determined on the sample plane 20 by deflecting the beam by using the objective deflector 18.
In the above-described apparatus, when the electron beam irradiation position is moved to a position distanced from a center of the object lens 19, there occurs a problem that an image forming position deviates from the sample plane 20 toward the upstream side by the field curvature aberration. Similarly, as shown in FIG. 2, even when transferring a large pattern (character pattern) on an EB (Electron Beam) mask 80 onto the sample plane 20 by scanning this pattern with the electron beam, there is produced a problem that a focusing position deviates in accordance with a distance from the central axis on the pattern. Further, the sample plane is not necessarily always flat, and a height of the sample plane differs depending on a position on the sample pattern. Therefore, there may occurs a problem that a focal point may deviate in accordance with a position on the sample plane.
As a countermeasure for this problem, the image forming position can be adjusted to be placed on the sample plane 20 by varying the excitation of the object lens 19 in accordance with a deflection position on the sample plane 20. However, the object lens 18 is generally an electromagnetic lens, and it is difficult to change the excitation at a high speed.
On the other hand, as shown in FIG. 3, correction can be performed by providing an axial symmetric electrostatic lens 90 in the middle of a beam axis (optical axis). However, there is a restriction in design such that the independent axial symmetric electrostatic lens can not be placed at the same position as the deflector. Furthermore, Published Japanese Patent No. 2946537 discloses a method by which the entire potentials of the electrostatic deflector are simultaneously changed so that the deflector can be substantially used as the axial symmetric electrostatic lens. According to this method, the energy of the electrons is locally changed by varying the potentials so that the focal distance is changed. This method, however, has a drawback that the sensitivity for changing the focal distance is low.
As described above, in the conventional electron beam exposing apparatus, when the electron beam is moved to a position distanced from the center of the object lens, there occurs a problem that the image forming position deviates from the sample plane toward the upstream side by the field curvature aberration. Even in the case that a large pattern on the EP mask is transferred onto the sample plane by scanning this pattern by using the electron beam, the focal position is disadvantageously shifted depending on a distance from the central axis on the pattern. Therefore, such a dynamic focal point must be corrected.
Although an image forming position can be adjusted to be positioned on the sample by changing the excitation of the object lens in accordance with a deflection position of the sample, it is difficult to change the excitation at a high speed in case of the object lens including an electromagnetic lens. Moreover, although correction can be performed by providing the axial symmetric electrostatic lens, the axial symmetric electrostatic lens can not be placed at the same position as the deflector, which results in increase in the optical system. In addition, the axial symmetric electrostatic lens has a drawback that the sensitivity for changing a focal distance is low.
Additionally, these problems are not restricted to the electron beam exposing apparatus, and an ion beam exposing apparatus also has such problems.
Thus, it is necessary to realize a charged beam exposing apparatus which can perform dynamic focus correction for a charged particle beam at a high speed and improve the accuracy in lithography.
According to a first aspect of the present invention, there is provided a charged particle beam exposing apparatus comprising:
a charged particle beam gun;
a first shaping aperture which forms a charged particle beam from the beam gun into a predetermined shape and passes it therethrough;
a first projection lens provided on a downstream side of the first shaping aperture and passes it therethrough;
a shaping deflector which deflects the charged particle beam which passes through the first projection lens;
a second projection lens on which the charged particle beam which passes through the shaping deflector is incident;
a second shaping aperture which forms the charged particle beam which passes through the second projection lens into a predetermined shape and passes it therethrough;
a reduction lens on which the charged particle beam which passes through second shaping aperture is incident;
an objective deflector which is provided on the downstream side of the reduction lens and deflects the charged particle beam which passes through the reduction lens;
an object lens on which the charged particle beam which passes through the objective deflector is incident;
a sample base provided on the downstream side of the object lens; and
at least two pairs of quadrupole field generators which are provided along a beam axis between the second shaping aperture and the sample base so as to give the focusing force to the charged particle beam and generate a quadrupole field, a field intensity of the quadrupole field generators being variable in accordance with a deflection position of the charged particle beam on a sample mounted on the sample base in order to correct a focusing position of he charged particle beam.
According to a second aspect of the present invention, there is provided a charged particle beam exposing apparatus comprising:
a charged particle beam gun;
a shaping aperture which forms a charged particle beam from the beam gun into a predetermined shape and passes it therethrough;
a first projection lens which is provided on a downstream side of the shaping aperture;
a shaping deflector which deflects the charged particle beam which passes through the first projection lens;
a second projection lens on which the charged particle beam which passes through the shaping deflector is incident;
a mask which forms the charged particle beam which passes through the second projection lens into a predetermined pattern and passes it therethrough;
a reduction lens on which the charged particle beam which passes through the mask is incident;
an objective deflector which is provided on the downstream side of the reduction lens and deflects the charged particle beam;
an object lens on which the charged particle beam which passes through the objective deflector is incident;
a sample base provided on a downstream side of the object lens; and
at least two pairs of quadrupole field generators which are provided along a beam axis between the second shaping aperture and the sample base so as to give the converging force to the charged particle beam and generate a quadrupole field, a field intensity of the quadrupole field generators being variable in accordance with a deflection position of the charged particle beam on a sample mounted on the sample base in order to correct a focusing position of the charged particle beam.
In the charged particle beam exposing apparatus according to the second aspect, the quadrupole field generators may be provided between the shaping aperture and the mask.